Prior to Bane, there were as many Sith as there are Jedi. However, due to the inherent nature of the dark side, they would continually weaken their order. Groups of weaker Sith would team up to bring down stronger ones and then the process would repeat itself over and over. What you got was a dilution of the Sith order, where their numbers were many but their individual power level was continually diminishing from generation to generation.

Enter the rule of two: "Two there should be; no more, no less. One to embody the power, the other to crave it." Now the apprentice must grow more powerful than the master, kill him/her and take on an apprentice of his/her own. If the apprentice fails, he or she is struck down and a new apprentice is found. Thus each subsequent Sith generation gets stronger and stronger; a concentration of power. That's the theory anyways.

So it is not really a case of backstabbing. The apprentice is expected and indeed trained to eventually their shot at the master.

Prior to Bane, there were as many Sith as there are Jedi. However, due to the inherent nature of the dark side, they would continually weaken their order. Groups of weaker Sith would team up to bring down stronger ones and then the process would repeat itself over and over. What you got was a dilution of the Sith order, where their numbers were many but their individual power level was continually diminishing from generation to generation.

Enter the rule of two: "Two there should be; no more, no less. One to embody the power, the other to crave it." Now the apprentice must grow more powerful than the master, kill him/her and take on an apprentice of his/her own. If the apprentice fails, he or she is struck down and a new apprentice is found. Thus each subsequent Sith generation gets stronger and stronger; a concentration of power. That's the theory anyways.

So it is not really a case of backstabbing. The apprentice is expected and indeed trained to eventually their shot at the master.

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Indeed, although Darth Sidious broke the rule by training Darth Maul while he was still Plagueis' apprentice, as did Count Dooku during the Clone Wars with Asajj Ventress and Savage Oppress while he was still Sidious' apprentice. Also in Legends Darth Vader breaks the rule by training Galen Marek, AKA Starkiller, while he was Sidious' apprentice.

Indeed, although Darth Sidious broke the rule by training Darth Maul while he was still Plagueis' apprentice, as did Count Dooku during the Clone Wars with Asajj Ventress and Savage Oppress while he was still Sidious' apprentice. Also in Legends Darth Vader breaks the rule by training Galen Marek, AKA Starkiller, while he was Sidious' apprentice.

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I don't think Asajj Ventress was ever a full on Sith apprentice. But you are correct that there are definitely exceptions, but order is usually restored.

Maybe he lost it or threw it away or buried it or something. We don't know much about what he even does on that planet. Maybe he even gave up using the force because he wants the Jedi to end. Much like Kyle Katarn did in one story IIRC.